Flat Roof Boarding and Sheathing: What Does the Structural Specification Require?

Flat roof boarding — the structural deck that goes over the roof joists and forms the base for waterproofing — sits in the overlap between structural engineering and building physics. The structural engineer sizes the roof joists. The architect specifies the sheathing board as part of the roof build-up section.

Who Specifies the Boarding?

In almost all cases, the architect specifies the flat roof boarding as part of their Building Regulations section drawings. The structural engineer’s role is to size the joists — the boarding type and thickness is an architectural and performance specification, not a structural calculation item.

If there is no architect on your project, you and the contractor are responsible for specifying the boarding to comply with Building Regulations.

LABC Recommendation

As a general guide, the London and Area Building Control (LABC) recommendation for flat roof decking is:

A single layer of minimum 18 mm structural plywood or OSB3 (oriented strand board, exposure grade) laid with joints staggered in both directions.

This recommendation provides adequate structural deck performance for domestic flat roofs with standard joist spacing (up to 400 mm) and domestic loading.

Check With Your Building Control Officer

Before purchasing boarding, confirm the specification with your BCO. Some officers have specific preferences — for example:

– Some require a minimum 18 mm per layer; others accept 15 mm if joists are at 400 mm centres
– Some accept OSB3; others prefer plywood
– Some require a vapour control layer to be specified before the roof build-up is finalised

Getting this wrong before the boarding is ordered and fixed wastes money and time. A five-minute call to Building Control before ordering is worthwhile.

What PorthouseDean Specifies

PorthouseDean specifies the structural joists — their span, depth, grade, and spacing. If a structural diaphragm function is required from the roof deck (for example, to transfer wind loads from a high parapet to the structure below), we would specify this as part of the structural design. In most standard domestic flat roof situations, the deck specification is left to the architect.

Contact us if your structural package requires a specific boarding or diaphragm specification.